Protective film for a polarizer, a polarizing plate comprising the same, and a display device with the polarizing plate

ABSTRACT

Disclosed are a protective film for a polarizer with superior optical and mechanical properties, a polarizing plate including the same and a display device including the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims, under 35 U.S.C. § 119, the priority of KoreanPatent Application No. 10-2016-0076724, filed on Jun. 20, 2016, in theKorean Intellectual Property Office, the entire contents of which areincorporated herein by reference.

BACKGROUND (a) Technical Field

The present invention relates to a protective film for a polarizer withsuperior optical and mechanical properties, a polarizing plate includingthe same and a display device including the same.

(b) Background Art

Recently, interests in a polarizing plate, an essential component of aliquid crystal display (LCD), are increasing as the demand on the liquidcrystal display increases rapidly.

The polarizing plate, which polarizes incident natural light oscillatingin various directions into light oscillating in one direction only, isan essential component for providing transmitted light and changing thecolor tone of the transmitted light.

The polarizing plate has a structure in which a protective film isstacked on one or both sides of a polarizer. As the polarizer, polyvinylalcohol (PVA) is commonly used. In the past, triacetyl cellulose (TAC)was commonly used as the protective film.

Meanwhile, as the functions and applications of the liquid crystaldisplay (LCD) become more diversified, normal operation under harshconditions is required. However, triacetyl cellulose (TAC) does notsatisfy this requirement because it is vulnerable to moisture and hasweak durability.

Recently, there have been many attempts to replace the triacetylcellulose (TAC) with polyethylene terephthalate (PET) as in JapanesePatent Application No. 2011-532061 and Japanese Patent Publication No.2010-118509. It is because polyethylene terephthalate (PET) can satisfythe above requirement because it has superior mechanical property,chemical resistance and moisture barrier property.

However, because polyethylene terephthalate (PET) is highlybirefringent, it leads to distorted polarization between the polarizerand the liquid crystal and, accordingly, significantly reducesvisibility. A typical example is the rainbow stains appearing on thesurface of the protective film.

Because the rainbow stains become easily visible due to the recent trendof high brightness and high color purity of the liquid crystal display(LCD), the rainbow stains are a big obstacle to use of the polyethyleneterephthalate (PET) for the protective film.

REFERENCES OF THE RELATED ART Patent Documents

-   (Patent document 1) Japanese Patent Application No. 2011-532061.-   (Patent document 2) Japanese Patent Publication No. 2010-118509.

SUMMARY

The present invention has been made to resolve the above-describedproblems and limitations.

The present invention is directed to providing a protective film for apolarizer free from rainbow stains, a polarizing plate including thesame and a display device including the same.

The present invention is also directed to providing a protective filmwhich has good mechanical property such as degree of crystallization,tensile strength, pencil hardness, etc. without impairing visibility dueto superior optical property, a polarizer including the same and adisplay device including the same.

The purposes of the present invention are not limited to those mentionedabove. The purposes of the present invention will become more apparentby the following description and will be realized by the means describedin the claims and their combinations.

A protective film for a polarizer according to an exemplary embodimentmay contain polyethylene terephthalate (PET), satisfy the conditions of(1) and (2):

(1) in-plane phase difference (R_(o))≤350 nm

(2) 16,000 nm phase difference in thickness direction (R_(th))≥6,000 nm,

and experience change in the phase difference in the thickness directionwith respect to the width change within the effective width(|ΔR_(th)|/|Δx|) of less than 1.5 nm/mm.

The protective film for a polarizer according to an exemplary embodimentmay have a phase difference in the thickness direction (R_(th)) at thewidth center of 6,800 nm or more.

The protective film for a polarizer according to an exemplary embodimentmay have a ratio (R_(th)/R_(o)) of the phase difference in the thicknessdirection (R_(th)) with respect to the in-plane phase difference (R_(o))at the width center of 60 or greater.

The protective film for a polarizer according to an exemplary embodimentmay experience variation in the phase difference in the thicknessdirection (R_(th,max)−R_(th,min)) within the effective width of 1,500nm/m or less.

The protective film for a polarizer according to an exemplary embodimentmay have an in-plane phase difference (R_(o)) at the width center of 200nm or less.

The protective film for a polarizer according to an exemplary embodimentmay have an in-plane phase difference (R_(o)) within ±500 mm from thewidth center along the width direction of 250 nm or less.

The protective film for a polarizer according to an exemplary embodimentmay have an in-plane phase difference (R_(o)) within ±1000 mm from thewidth center along the width direction of 300 nm or less.

The protective film for a polarizer according to an exemplary embodimentmay experience variation in the in-plane phase difference(R_(o,max)−R_(o,min)) within the effective width of 250 nm/m or less.

The protective film for a polarizer according to an exemplary embodimentmay experience change in the in-plane phase difference with respect tothe width change (|ΔR_(o)|/|Δx|) within the effective width of less than0.3 nm/mm.

The protective film for a polarizer according to an exemplary embodimentmay have a stretching ratio in the length direction (MD) of 2.8-3.5times and a stretching ratio in the width direction (TD) of 2.9-3.7times.

The protective film for a polarizer according to an exemplary embodimentmay have a ratio (MD/TD) of a stretching ratio in the length direction(MD) with respect to a stretching ratio in the width direction (TD) of0.9-1.1.

The protective film for a polarizer according to an exemplary embodimentmay have a thickness of 20-60 μm.

The protective film for a polarizer according to an exemplary embodimentmay be heat-set at 160-230° C.

A polarizing plate according to an exemplary embodiment may include apolarizer and the protective film for a polarizer which is adjacent toat least one of the upper side and the lower side of the polarizer.

A display device according to an exemplary embodiment may include adisplay panel and the polarizing plate which is disposed on at least oneof the upper side and the lower side of the display panel.

The present invention provides the following advantageous effects.

The protective film for a polarizer according to an exemplary embodimentand the polarizing plate including the same do not impair visibilitybecause they are free from rainbow stains and have good durability dueto superior mechanical property such as tensile strength, pencilhardness, etc.

Accordingly, the display device equipped with the polarizing plateaccording to an exemplary embodiment can be used in various applicationsbecause it has superior optical property and can be operated normallyeven under harsh environment.

The effects of the present invention are not limited to those describedabove. It is to be understood that the effects of the present inventioninclude all the effects that can be inferred from the followingdescription.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows a polarizing plate according to an exemplaryembodiment.

FIG. 2 is a diagram for describing a protective film for a polarizeraccording to an exemplary embodiment.

FIG. 3 schematically shows a liquid crystal display as an exemplarydisplay panel equipped with a polarizing plate according to an exemplaryembodiment.

FIG. 4 schematically shows an organic electroluminescence display as anexemplary display panel equipped with a polarizing plate according to anexemplary embodiment.

FIGS. 5A-5C show a result of measuring in-plane phase difference (R_(o))for the entire effective width of a protective film of Example 1. FIGS.5A, 5B and 5C show results for the entire effective width, the rangefrom 0 mm (width center) to −1,500 mm, and the range from 0 mm (widthcenter) to +1,500 mm, respectively.

FIGS. 6A-6C show a result of measuring phase difference in the thicknessdirection (R_(th)) for the entire effective width of a protective filmof Example 1. FIGS. 6A, 6B and 6C show results for the entire effectivewidth, the range from 0 mm (width center) to −1,500 mm, and the rangefrom 0 mm (width center) to +1,500 mm, respectively.

DETAILED DESCRIPTION OF MAIN ELEMENTS

10: polarizing plate 11: polarizer 12: protective film for polarizer

DETAILED DESCRIPTION

Hereinafter, the present invention is described in detail by exemplaryembodiments. The exemplary examples can be modified in various formswithin the scope of the present invention and scope of the presentinvention is not limited by the exemplary embodiments.

In the exemplary embodiments described below, a film, membrane, panel,layer, etc. formed “on” or “under” a film, membrane, panel, layer, etc.may be formed either “directly” or “indirectly with another componentdisposed therebetween”.

In the attached drawings, the components may be magnified in size forthe purpose of illustration.

FIG. 1 schematically shows a polarizing plate 10 according to anexemplary embodiment.

The polarizing plate 10 according to an exemplary embodiment includes apolarizer 11 and a protective film 12 for a polarizer (hereinafter,‘protective film’) which is adjacent to at least one of the upper sideand the lower side of the polarizer 11.

The polarizer 11 polarizes natural light incident on the polarizingplate while oscillating in various directions into light oscillating inone direction only. The polarizer may be polyvinyl alcohol (PVA) dopedwith iodine, etc.

Polyvinyl alcohol (PVA) molecules contained in the polarizer may bearranged along one direction.

Specifically, the protective film 12 may be formed from a materialhaving superior mechanical property. Accordingly, the protective filmmay be formed from a material having polyester as a main component. Bycrystallizing the polyester by performing heating, stretching, etc.,degree of crystallization can be increased and, through this, mechanicalproperty such as tensile strength, etc. may be enhanced.

In addition, the polyester may improve the durability of the polarizingplate under humid environment because it has lower water vaporpermeability than triacetyl cellulose (TAC).

As the polyester, a homopolymer obtained from polycondensation of adicarboxylic acid such as terephthalic acid, isophthalic acid,ortho-phthalic acid, 2,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid,1,5-naphthalenedicarboxylic acid, diphenylcarboxylic acid,diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid,anthracenedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid,1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid,hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid,dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaricacid, 2,2-dimethylglutaric acid, adipic acid, 2-methyladipic acid,trimethyladipic acid, pimelic acid, azelaic acid, dimer acid, sebacicacid, suberic acid, dodecanedicarboxylic acid, etc. or a diol such asethylene glycol, propylene glycol, hexamethylene glycol, neopentylglycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol,decamethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-oebtabediol,1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane,bis(4-hydroxyphenyl)sulfone, etc., a copolymer obtained frompolycondensation of one or more dicarboxylic acid and two or more diols,a copolymer obtained from polycondensation of two or more dicarboxylicacids and one or more diol or a blend resin obtained from blending oftwo or more of the homopolymer or the copolymer may be used.

Specifically, an aromatic polyester may be used considering the degreeof crystallization of the polyester. Most specifically, polyethyleneterephthalate (hereinafter, ‘PET’) may be used.

However, PET may not be suitable for use as the protective film becauseit lacks crystallinity in an unstretched state. Accordingly, biaxiallystretched PET may be used as the protective film.

The PET may be biaxially stretched along a width direction (transversedirection, TD) and a length direction (machine direction, MD) throughsimultaneous biaxial stretching or sequential biaxial stretching.Specifically, the PET may be biaxially stretched sequentially bystretching along one direction and then stretching along a directionperpendicular thereto, although not being limited thereto.

Although the PET has superior mechanical property and moisture barrierproperty, it may distort polarization when used in the protective filmas it is due to very high birefringence. A typical example is therainbow stains described above.

Accordingly, in the present invention, the optical property of the PETis improved to prevent the rainbow stains so that it is suitable to beused for the protective film. A detailed description is given below.

The protective film satisfies the conditions of (1) and (2).

(1) In-plane phase difference (R_(o))≤350 nm

(2) Phase difference in thickness direction (R_(th))≥6,000 nm

The in-plane phase difference (R_(o)) is a parameter defined as theanisotropy of refractive indices (ΔN_(xy)=|N_(x)−N_(y)|) in twoperpendicular axes on the protective film (see FIG. 2) times thethickness d of the protective film, i.e., ΔN_(xy)×d, and is a measure ofoptical isotropy and anisotropy.

The phase difference in the thickness direction (R_(th)) is a parameterrepresenting the mean of phase difference which is obtained by twobirefringences ΔN_(xz)(=|N_(x)−N_(z)|) and ΔN_(yz)(=|N_(y)−N_(z)|) seenfrom the cross section of the protective film times the thickness d ofthe protective film.

Specifically, the in-plane phase difference (R_(o)) of the protectivefilm may be 350 nm or less. If the in-plane phase difference (R_(o))increases, the occurrence of rainbow stains becomes severe. Therefore,the smaller the in-plane phase difference, the better. However, becausestretching ratio or thickness has to be decreased to reduce the in-planephase difference of PET, mechanical property may worsen. Accordingly,for balanced optical property and mechanical property, the lower limitof the in-plane phase difference (R_(o)) may be 10 nm or more,specifically 30 nm or more, more specifically 50 nm or more.

As described above, it is easier to prevent the occurrence of rainbowstains as the in-plane phase difference (R_(o)) is smaller. Accordingly,the in-plane phase difference (R_(o)) at the width center of theprotective film may be 200 nm or less.

In the present disclosure, the ‘width center’ is defined as the midpoint(A, B) of the width the protective film after being stretched in thewidth direction (TD) and the length direction (MD), as shown in FIG. 2.The width center is not present singularly in the protective film butmay be present in numerous numbers depending on the measurement site.

And, the ‘effective width’ which will be described below refers to thelength in the width direction required for the protective film to beapplicable for a polarizing plate for large-screen applications.Specifically, it refers to the distance between the positions (A′, A″)that have been moved from the width center (A) along the x-axis towardboth ends, as shown in FIG. 2. In an exemplary embodiment, it is definedas ±1,500 mm from the width center, i.e., about 3,000 mm.

The protective film may have a variation in the in-plane phasedifference (R_(o,max)−R_(o,min)) within the effective width of 250 nm/mor less, more specifically 167 nm/m or less. The variation in thein-plane phase difference is the difference between the maximum(R_(o,max)) and the minimum (R_(o,min)) of the in-plane phase differenceper meter (m) within the effective width. If the variation in thein-plane phase difference is small, the occurrence of rainbow stains canbe prevented effectively because the in-plane phase difference (R_(o))does not increase significantly even when the width of the protectivefilm is large.

The protective film may experience change in the in-plane phasedifference with respect to the width change within the effective width(|ΔR_(o)|/|Δx|) of less than 0.3 nm/mm. The width change refers to thedistance between given points along the x-axis (Δx=x₂−x₁) and the changein the in-plane phase difference refers to the change in the in-planephase difference at the given points (ΔR_(o)=Ro,₂−Ro,₁). By controllingsuch that the change in the in-plane phase difference with respect tothe width change is small, the in-plane phase difference (R_(o)) may beprevented from increasing significantly within the effective width.

Accordingly, it is desired that the protective film has the in-planephase difference (R_(o)) at the width center of 200 nm or less, thein-plane phase difference (R_(o)) within ±500 mm from the width centeralong the width direction of 250 nm or less and the in-plane phasedifference (R_(o)) within ±1000 mm from the width center along the widthdirection of 300 nm or less while satisfying the condition (1).

Specifically, the protective film may have a phase difference in thethickness direction (R_(th)) of 6,000 nm or more. If the phasedifference in the thickness direction (R_(th)) is large, crystallizationis accelerated because the degree of molecular orientation in theprotective film is large. Therefore, it is desired that the phasedifference in the thickness direction (R_(th)) is large in the aspect ofmechanical property. In addition, as the phase difference in thethickness direction (R_(th)) is larger, the ratio (R_(th)/R_(o)) of thephase difference in the thickness direction (R_(th)) with respect to thein-plane phase difference (R_(o)) at the width center to be describedbelow becomes larger. Accordingly, the rainbow stains can be preventedeffectively. But, for PET, the thickness has to be increased to increasephase difference in the thickness direction (R_(th)), which isdisadvantageous in terms of cost and film thickness. Accordingly, theupper limit of the phase difference in the thickness direction (R_(th))may be set to 16,000 nm or less, specifically 15,000 nm or less, morespecifically 14,000 nm or less.

As described above, as the phase difference in the thickness direction(R_(th)) is larger, it is easier to prevent the occurrence of rainbowstains and improve mechanical property. Accordingly, the protective filmmay have the phase difference in the thickness direction (R_(th)) at thewidth center of 6,800 nm or more.

And, for the same reason as the in-plane phase difference (R_(o)), it isdesired that the protective film has the phase difference in thethickness direction within the effective width (R_(th,max)−R_(th,min))of 1,500 nm/m less, more specifically 1,000 nm/m or less, and the changein the phase difference in the thickness direction with respect to thewidth change within the effective width (|ΔR_(th)|/|Δx|) of less than1.5 nm/mm. The width change refers to the distance between given pointsalong the x-axis (Δx=x₂−x₁) and the phase difference in the thicknessdirection refers to the phase difference in the thickness direction atthe between given points (ΔR_(th)=R_(th,2)−R_(th,1)).

In addition to satisfying the in-plane phase difference (R_(o)) andphase difference in the thickness direction (R_(th)) conditionsdescribed above, the protective film may have the ratio (R_(th)/R_(o))of the phase difference in the thickness direction (R_(th)) with respectto the in-plane phase difference (R_(o)) at the width center of 30 orlarger, specifically 50 or larger, more specifically 60 or larger.Because it is easier to prevent the occurrence of rainbow stains as thein-plane phase difference (R_(o)) is smaller and the phase difference inthe thickness direction (R_(th)) is larger, it is desired to maintainthe ratio of the two values (R_(th)/R_(o)) large.

The present invention provides the protective film which has improvedoptical property while maintaining the superior mechanical property ofPET to be applicable to a polarizing plate used in various applications.Hereunder is given a detailed description.

Specifically, the protective film may have a degree of crystallizationof 35-55%. If the degree of crystallization is less than 35%, mechanicalproperty such as tensile strength, etc. may be unsatisfactory. And, ifit exceeds 55%, the protective film may break easily due to excessivedegree of crystallization.

The degree of crystallization (X_(c)) is calculated by Equation 1.X _(c) [%]=d _(c)(d−d _(a))/d(d _(c) −d _(a))*100  [Equation 1]

X_(c): degree of crystallization, d_(c): density (g/cm³) of crystallineregion, d_(a): density (g/cm³) of amorphous region, d: density (g/cm³)at measured site

In an exemplary embodiment, d_(c) and d_(a) are calculated as 1.455g/cm³ and 1.335 g/cm³, respectively.

Specifically, the protective film may have a pencil hardness of 5B orgreater. If the pencil hardness is 6B or lower, it may be difficult toprotect a polarizer from external force. In an exemplary embodiment, theprotective film may further include a hard coating layer on thepolarizer. The polarizer further including the hard coating layer mayhave a pencil hardness of 1H or greater.

The protective film may have a tensile modulus at high temperature (85°C.) of 3.0 GPa or greater, more specifically 3.5 GPa or greater.

The protective film is heat-treated after it is introduced to apolarizing plate. If the tensile modulus of the protective film at hightemperature (85° C.) is 3.0 GPa or greater, the polarizing plate may beprevented from curling.

Specifically, the polyvinyl alcohol (PVA) used as the polarizer, itcurls easily during the heat treatment because of high shrinkage. Ifthis is not prevented, wave patterns may occur on the protective filmand visibility may be impaired significantly due to glittering. Becausethe protective film has a high tensile modulus at high temperature (85°C.), the curling of the polyvinyl alcohol (PVA) can be prevented and,therefore, the wave patterns, glittering, separation of the polarizerfrom the protective film, cracking, etc. can be prevented in advance.

In the present invention, the protective film may be formed as follows.

The protective film may be formed by stretching an unstretched sheetformed of PET 2.8-3.5 times in the length direction (MD) and 2.9-3.7times in the width direction (TD).

The protective film may have similar stretching ratios in the lengthdirection (MD) and in the width direction (TD). Therefore, the ratio(MD/TD) of the stretching ratio in the length direction (MD) to thestretching ratio in the width direction (TD) may be 0.9-1.1.

And, the protective film may be formed by stretching in the lengthdirection (MD) and in the width direction (TD) at a stretching speed of6.5-8.5 m/min, although not being limited thereto.

The protective film may be preheated to a predetermined temperaturebefore stretching in the length direction (MD) and in the widthdirection (TD). Specifically, the preheating temperature may be in therange from T_(g)+5° C. to T_(g)+50° C. Although stretchability is goodas the T_(g) is lower, fracture may occur. Therefore, the stretching maybe performed after preheating to about 78° C.

The protective film formed by stretching under the above condition mayhave a thickness of 40-60 μm. Also, the protective film may be fixedthrough heat treatment after the stretching is completed. The heattreatment may be performed at 160-230° C.

The polarizing plate according to an exemplary embodiment may be appliedfor a display device such as a liquid crystal display, an organicelectroluminescence display, etc.

The display device includes a display panel and the polarizing platewhich is disposed on at least one of the upper side and the lower sideof a display panel.

FIG. 3 schematically shows a liquid crystal display as an exemplarydisplay panel equipped with a polarizing plate according to an exemplaryembodiment.

The liquid crystal display includes a liquid crystal panel 70 and abacklight unit 80.

The backlight unit 80 emits light to the liquid crystal panel 70. Theliquid crystal panel 70 displays images using the light incident fromthe backlight unit.

The liquid crystal panel 70 includes an upper polarizing plate 10, acolor filter substrate 71, a liquid crystal layer 72, a TFT substrate 73and a lower polarizing plate 10′.

The TFT substrate 73 and the color filter substrate 71 face each other.

The TFT substrate 73 may include a plurality of electrodes correspondingto respective pixels, a thin-film transistor connected to theelectrodes, a plurality of gate wirings applying driving signals to thethin-film transistor and a plurality of data wirings applying datasignals to the electrodes through the thin-film transistor.

The color filter substrate 71 includes a plurality of color filtercorresponding to respective pixels. The color filter creates red, greenand blue colors by filtering incident light. The color filter substratemay include a common electrode facing the electrodes.

The liquid crystal layer 72 is interposed between the TFT substrate andthe color filter substrate. The liquid crystal layer may be driven bythe TFT substrate. More specifically, the liquid crystal layer may bedriven by an electric field formed between the electrodes and the commonelectrode. The liquid crystal layer may control the polarizationdirection of the light passing through the polarizing plate therebelow.That is to say, the TFT substrate may control the potential differenceapplied between the electrodes and the common electrode in pixel units.Accordingly, the liquid crystal layer may be driven to have differentoptical properties in pixel units.

The upper polarizing plate 10 is disposed on the color filter substrate71. The upper polarizing plate 10 may be adhered to the upper side ofthe color filter substrate 71.

The lower polarizing plate 10′ is disposed below the TFT substrate 73.The lower polarizing plate 10′ may be adhered to the lower side of theTFT substrate 73.

The polarization directions of the upper polarizing plate 10 and thelower polarizing plate 10′ may be identical or perpendicular to eachother.

FIG. 4 schematically shows an organic electroluminescence display as anexemplary display panel equipped with a polarizing plate according to anexemplary embodiment.

The organic electroluminescence display includes a front polarizingplate 10 and an organic EL panel 90.

The front polarizing plate 10 may be disposed on the front side of theorganic EL panel 90. More specifically, the front polarizing plate maybe attached the side of the organic EL panel on which images aredisplayed. The front polarizing plate may have substantially the sameconstitution as the polarizing plate descried above.

The organic EL panel displays images through luminescence by the pixelunits. The organic EL panel includes an organic EL substrate 91 and adriving substrate 92.

The organic EL substrate 91 includes a plurality of organicelectroluminescence units corresponding to respective pixels. Each ofthe organic electroluminescence units includes a cathode, an electrontransport layer, a luminescence layer, a hole transport layer and ananode. Detailed description of the cathode, etc. will be omitted.

The driving substrate 92 is drivably coupled to the organic EL substrate31. That is to say, the driving substrate may be coupled to the organicEL substrate so as to apply driving signals such as driving current,etc. More specifically, the driving substrate may drive the organic ELsubstrate by applying current to the respective organicelectroluminescence units.

EXAMPLES

The present invention will be described in more detail through examples.The following examples are for illustrative purposes only and it will beapparent to those skilled in the art that the scope of this invention isnot limited by the examples.

Examples 1-3 and Comparative Examples 1-4

A polyethylene terephthalate (PET) resin (SKC) was used as a material ofa protective film. An unstretched sheet was prepared by extruding thePET resin at about 280° C. using an extruder and casting at about 30° C.using a casting roll.

After preheating, the unstretched sheet was stretched at 125° C. in thelength direction (MD) and the width direction (TD) with the stretchingratio described in Table 1. Then, a protective film was prepared by heatsetting the stretched sheet at the temperature described in Table 1 forabout 30 seconds.

TABLE 1 MD stretching MD TD ratio/TD Preheating Heat setting Thicknessstretching stretching stretching temperature temperature [μm] ratioratio ratio [° C.] [° C.] Example 1 40 3.3 times 3.5 times 0.94 78 180Example 2 40 3.1 times 3.4 times 0.91 78 230 Example 3 50 3.1 times 3.4times 0.91 78 230 Comparative 30 3.1 times 3.4 times 0.91 78 230 Example1 Comparative 50 3.2 times 4.2 times 0.76 78 230 Example 2 Comparative80 1.2 times 4.3 times 0.28 78 210 Example 3 Comparative 40 3.2 times3.6 times 0.89 78 230 Example 4

Measurement Example

The in-plane phase difference (R_(o)), phase difference in the thicknessdirection (R_(th)), in-plane phase difference within the effective width(R_(o,max)−R_(o,min)) and phase difference in the thickness directionwithin the effective width (R_(th,max)−R_(th,min)) of the protectivefilms prepared in the examples and comparative examples were measured.The result is given in Table 2.

The in-plane phase difference (R_(o)) and the phase difference in thethickness direction (R_(th)) were measured as follows.

After determining the orientation axis of the protective film using twosheets of the polarizing plate, a sample for measurement was prepared bycutting to a size of 4 cm×2 cm perpendicularly to the orientation axis.The in-plane phase difference and the phase difference in the thicknessdirection were measured using a phase difference meter (Axometrics,Axoscan, measured at 550 nm). The refractive index of the protectivefilm (sample) was measured using an Abbe refractometer (Atago, NAR-4T,measured at 546 nm) and the thickness d (μm) of the protective film wasmeasured using an electronic micrometer (Fineloop, Millitron 1245D).

The in-plane phase difference (R_(o)) and the phase difference in thethickness direction (R_(th)) were measured for the protective film ofExample 1 over the entire effective width. The result is shown in FIGS.5A-5C and FIGS. 6A-6C.

TABLE 2 R_(o) ¹⁾ Variation of R_(o) within [nm] R_(th) ²⁾ [nm]R_(th)/R_(o) ³⁾ [nm] effective width⁴⁾ [nm/m] Example 1 98 6,850 69.9250 Example 2 170 6,100 35.9 320 Example 3 160 8,200 51.3 460Comparative 130 5,700 43.8 270 Example 1 Comparative 1900 10,400 5.47420 Example 2 Comparative 8100 7,100 0.88 100 Example 3 Comparative 3055,500 18.0 410 Example 4 ¹⁾In-plane phase difference (R_(o)) at widthcenter ²⁾Phase difference in thickness direction (R_(th)) at widthcenter ³⁾Ratio of phase difference in thickness direction (R_(th)) withrespect to in-plane phase difference (R_(o)) at width center ⁴⁾Effectivewidth: ±1,500 mm from width center (about 3,000 mm)

Referring to FIGS. 5A-5C, it can be seen that the protective film ofExample 1 satisfies the following conditions.

-   -   In-plane phase difference (R_(o)) at the width center ≤100 nm    -   In-plane phase difference (R_(o)) within ±500 mm from the width        center in the width direction ≤160 nm    -   In-plane phase difference (R_(o)) within ±1000 mm from the width        center in the width direction ≤300 nm    -   Change in the in-plane phase difference with respect to the        width change (|ΔR_(o)|/|Δx|) within the effective width <0.3        nm/mm

Referring to FIGS. 6A-6C, it can be seen that the protective film ofExample 1 satisfies the following conditions.

-   -   Phase difference in the thickness direction (R_(th)) at the        width center 6,800 nm    -   Change in the phase difference in the thickness direction with        respect to the width change (|ΔR_(th)|/|Δx|) within the        effective width <1.5 nm/mm

Test Example

The appearance after application to a display device, degree ofcrystallization, density, pencil hardness, pencil hardness after hardcoating and tensile modulus at high temperature of the protective filmsof the examples and comparative examples were evaluated. The result isgiven in Table 3 and Table 4.

The appearance was evaluated as follows.

The protective films of the examples and comparative examples wereintroduced to a polarizing plate with a structure shown in FIG. 1. Then,a hard coating layer was formed on the protective film. After applyingthe resulting polarizing plate to a TV or a monitor, it was visuallyevaluated whether rainbow stains or coloring from the front and obliquedirections of the polarizing plate.

⊚: No rainbow stain or coloring is observed from any direction.

◯: No rainbow stain is observed from any direction but very slightcoloring is observed from an oblique direction.

Δ: Slight rainbow stains and coloring are observed from an obliquedirection.

X: Clear rainbow stains and are observed from an oblique direction.

The degree of crystallization of the protective film was measured by thedensity method described above (Equation 1).

The pencil hardness of the protective film was measured using a pencilhardness tester (Kipae E&T, KP-M5000M) and a Mitsubishi ‘UNI’ gradepencil. The pencil hardness was also measured after forming the hardcoating layer on the protective film.

The tensile modulus of the protective film was measured using auniversal testing machine (Instron, 4485 TIC960203-97B1A).

TABLE 3 Degree of Pencil Appearance Appearance Density crystallizationPencil hardness on TV on monitor [g/cm³] [%] hardness after H/C Example1 ⊚ ⊚ 1.388 44 5B 2H Example 2 ◯ ⊚ 1.397 52 5B 2H Example 3 ⊚ ⊚ 1.397 525B 2H Comparative X ⊚ 1.398 53 6B 1H Example 1 Comparative X Δ 1.405 584B 2H Example 2 Comparative ⊚ ⊚ 1.397 43 5B 2H Example 3 Comparative X ⊚1.401 55 5B 2H Example 4

TABLE 4 Tensile modulus (@ 85° C.) Visibility¹⁾ MD [GPa] TD [GPa] Lengthdirection Width direction Example 1 3.7 3.8 ◯ ◯ Example 2 3.7 4.0 ◯ ◯Example 3 3.8 3.9 ◯ ◯ Comparative 4.0 4.1 X Δ Example 1 Comparative 3.84.2 ◯ ◯ Example 2 Comparative 2.2 5.8 X ◯ Example 3 Comparative 3.8 4.0◯ ◯ Example 4 ¹⁾Visibility was evaluated based on the occurrence of wavepatterns and glittering. The evaluation standard was as follows. ◯: nodecrease in visibility, Δ: slight decrease in visibility, X: severedecrease in visibility.

Referring to Table 3 and Table 4, it can be seen that the protectivefilms of Examples 1-3 can be used for various applications because theyshowed good degree of crystallization, pencil hardness and tensilemodulus while having superior optical property with no rainbow stain orcoloring.

In particular, it can be seen that the protective film of Example 1showed well-balanced optical property and mechanical property and, thus,is the most suitable for application to a polarizing plate for a displaydevice.

The present invention has been described in detail with reference tospecific embodiments thereof. However, it will be appreciated by thoseskilled in the art that various changes and modifications may be made inthese embodiments without departing from the principles and spirit ofthe invention, the scope of which is defined in the appended claims andtheir equivalents.

The polarizing plate having the protective film for a polarizeraccording to an exemplary embodiment is applicable to various displaydevices such as a liquid crystal display, an organic electroluminescencedisplay, etc.

What is claimed is:
 1. A protective film for a polarizer, the protectivefilm comprising polyethylene terephthalate (PET), wherein the protectivefilm satisfies the in-plane phase difference and phase difference in thethickness direction conditions of (1) and (2) at a wavelength of 550 nm:(1) 10 nm≤in-plane phase difference (R_(o))≤350 nm (2) 16,000 nm≥phasedifference in thickness direction (R_(th))≥6,000 nm, the protective filmexperiences change in the phase difference in the thickness directionwith respect to the width change within the effective width(|ΔR_(th)|/|Δx|) of less than 1.5 nm/mm, the protective film experiencesvariation in the phase difference in the thickness direction within theeffective width (R_(th,max)−R_(th,min)) of 1,500 nm/m or less, and theprotective film has a degree of crystallization (X_(c)) of 35-55%. 2.The protective film for a polarizer according to claim 1, which has aphase difference in the thickness direction (R_(th)) at the width centerof 6,800 nm or more.
 3. The protective film for a polarizer according toclaim 1, which has a ratio (R_(th)/R_(o)) of the phase difference in thethickness direction (R_(th)) with respect to the in-plane phasedifference (R_(o)) at the width center of 60 or larger and 1,600 orsmaller.
 4. The protective film for a polarizer according to claim 1,which has an in-plane phase difference (R_(o)) at the width center of200 nm or less.
 5. The protective film for a polarizer according toclaim 1, which has an in-plane phase difference (R_(o)) within ±500 mmfrom the width center along the width direction of 250 nm or less. 6.The protective film for a polarizer according to claim 1, which has anin-plane phase difference (R_(o)) within ±1000 mm from the width centeralong the width direction of 300 nm or less.
 7. The protective film fora polarizer according to claim 1, which experiences variation in thein-plane phase difference (R_(o,max)−R_(o,min)) within the effectivewidth of 250 nm/m or less.
 8. The protective film for a polarizeraccording to claim 1, which experiences change in the in-plane phasedifference with respect to the width change (|ΔR_(o)|/|Δx|) within theeffective width of less than 0.3 nm/mm.
 9. The protective film for apolarizer according to claim 1, which has a stretching ratio in thelength direction (MD) of 2.8-3.5 times and a stretching ratio in thewidth direction (TD) of 2.9-3.7 times.
 10. The protective film for apolarizer according to claim 1, which has a ratio (MD/TD) of astretching ratio in the length direction (MD) with respect to astretching ratio in the width direction (TD) of 0.9-1.1.
 11. Theprotective film for a polarizer according to claim 1, which has athickness of 20-60 μm.
 12. The protective film for a polarizer accordingto claim 1, which is stretched in the length direction and the widthdirection and then heat-treated at 160-230° C.
 13. A polarizing platecomprising: a polarizer; and the protective film for a polarizeraccording to claim 1, the protective film being adjacent to at least oneof the upper side and the lower side of the polarizer.
 14. A displaydevice comprising: a display panel; and the polarizing plate accordingto claim 13, the polarizing plate being disposed on at least one of theupper side and the lower side of the display panel.
 15. The protectivefilm for a polarizer according to claim 1, wherein the degree ofcrystallization (X_(c)) is calculated by:X _(c) [%]=d _(c)(d−d _(a))/d(d _(c) −d _(a))*100 where d_(c) is density(g/cm³) of crystalline region, d_(a) is density (g/cm³) of amorphousregion, and d is density (g/cm³) at measured site.
 16. The protectivefilm for a polarizer according to claim 15, wherein d_(c) is about 1.455g/cm³ and d_(a) is about 1.335 g/cm³.
 17. A polarizing plate comprising:a polarizer; a first protective film for a polarizer according to claim1, the first protective film being adjacent to the upper side of thepolarizer; and a second protective film for a polarizer according toclaim 1, the second protective film being adjacent to the lower side ofthe polarizer.
 18. A display device comprising: a display panel; a firstpolarizing plate according to claim 17, the first polarizing plate beingdisposed on the upper side of the display panel; and a second polarizingplate according to claim 17, the second polarizing plate being disposedon the lower side of the display panel.
 19. A display device comprising:a display panel; and the polarizing plate according to claim 17, thepolarizing plate being disposed on at least one of the upper side andthe lower side of the display panel.